Seismic base isolation by electro-osmosis during an earthquake event

ABSTRACT

A method and system of isolating a structure and soil mass from earthquake induced vibration by inducing soil liquefaction beneath a structure during an earthquake event, by monitoring local seismic precursor events, such as early arrival ground motion using an accelerometer, predicting the onset of a major earthquake tremor and energizing conductors in the ground by a dc power source for moving the ground water by electro-osmosis towards a suitable isolation layer, whereby raising the pore water pressure in the isolation layer and thus preferentially inducing localized soil liquefaction of the particular isolation layer during the earthquake event and thus isolating the structure and soil above the particular soil horizon from the upward propagating shear wave ground motions arising from the earthquake event.

TECHNICAL FIELD

This invention relates to seismic base isolation of a structure or soilmass from the vibratory motion generated during an earthquake, and moreparticularly to the preferential inducement of localized soilliquefaction of a particular isolation layer in the soil horizon beneaththe structure by applying an electro-osmotic gradient to the saturatedsoil during the earthquake event and thus raising the pore waterpressure to induce localized soil liquefaction within a particularisolation layer in the soil horizon which thus reduces the upwardpropagation of the earthquake induced shear wave ground motions to theoverlying structure.

BACKGROUND OF THE INVENTION

Earthquakes are caused by the resultant relative slippage of the earthcrust, generally along or near major tectonic plate boundaries. Incertain parts of the world, continuous differential movement occursbetween one section of the earth's crust and an adjacent one, causing anaccumulation of strain at the boundary. When the stresses caused by thisstrain accumulation exceed the strength of the earth's materials, a slipoccurs between two portions of the earth's crust and tremendous amountsof energy are released. This energy propagates outward from the focus ororigin of the earthquake in the form of body and surface elastic stresswaves.

The energy released during an earthquake event is transmitted throughthe earth's crust in the form of body and surface seismic waves. Thebody waves are composed of P-(compression) waves and S-(shear) waves,with the P-wave traveling significantly faster than the S-wave. Thesurface waves of most interest are the Rayleigh wave and the Love wave.The Love wave travels faster than the Rayleigh wave. The total energytransported is represented almost entirely by the Rayleigh, the S- andthe P-waves, with the Rayleigh wave carrying the largest amount ofenergy, the S-wave an intermediate amount, and the P-wave the least. Thevelocity of the P-wave is almost double that of the S-wave, and thevelocity of the S-wave is only slightly greater than the Rayleigh wave.

At some distance from an earthquake disturbance a particle at theearth's surface first experiences a displacement in the form of anoscillation at the arrival of the P-wave followed by a relatively quietperiod leading up to another oscillation at the arrival of the S- andRayleigh waves. These events are referred to as the minor tremor and themajor tremor at the time of arrival of the Rayleigh wave. The body andsurface waves are monitored during an earthquake to gauge theearthquake's intensity.

Earth ground motions experienced during an earthquake are actually quitecomplex due to the variation in the earth's crust, from strong stiffbedrock to soft weak soils. Considerable energy can be transmittedthrough the bedrock, and it appears that in many cases the main forcesacting on soil elements in the field during earthquakes are thoseresulting from the upward migration of shear motions from the underlyingrock formations. Although the actual wave pattern may be very complex,the resulting ground motion imposed on the soil and the overlyingstructure are predominantly from the upward propagation of the S-wavecomponents from the underlying bedrock. Deposits of thick soft soils cangive rise to amplification of these ground motions in particular in thelong period (low frequency) content of the earthquake induced shaking.Such amplification of earthquake induced ground motions can causeextensive damage to buildings, bridges, pipelines, embankments, dams,slopes, and other structures and works constructed on soft soildeposits.

The factors that effect the amplification of earthquake induced groundmotions are soil type, grain size distribution, compactness of the soil,thickness of the soil deposit, depth to groundwater, and the magnitudeand number of the strain reversals. Deposits of soft soils, such assilts and clays are most likely to amplify ground motions during anearthquake Structures constructed on such soils can be extensivelydamaged by even a moderate size earthquake. Two recent earthquakes, the1985 Michoacan (Mexico) and the 1989 Loma Prieta (Calif.), highlight theextensive earthquake induced damage to structures located on soft soildeposits. The 1985 Michoacan earthquake caused only moderate damage inthe vicinity of its epicenter but caused extensive damage to structureslocated on a thick deposit of soft silts and clay some 350 km away inMexico City. Likewise, the 1989 Loma Prieta earthquake caused minordamage in the vicinity of its epicenter but caused moderate to extensivedamage to structures located on the San Francisco Bay mud some 100 kmaway.

Conventional seismic isolation systems to minimize or prevent damage toa structure by isolating the structure from ground motions during anearthquake consist of the following:

1) sliding bearings with energy absorbing properties to isolate thestructure from horizontal earthquake induced ground motions, such aslead rubber, steel neoprene/rubber and fiber reinforced elastomer,

2) sliding bearings with fluid dampers to both isolate the structurefrom earthquake induced ground motion and modify the structural responseto minimize damage,

3) passive mass damping systems consisting of a pendulum suspendedweight and associated dampers to absorb vibratory energy and minimizedamage to the structure,

4) active mass damping systems consisting of a sensor and computercontrolled movement of a mass to minimize vibration and damage to thestructure,

5) pneumatic or fluidized foundation isolation system to reduceearthquake induced ground motions being transmitted to the structure.

The above methods have had mixed success in minimizing damage andvibrations to a structure during an earthquake. The passive and activemass damping systems have been shown to be successful during strongwinds and minor earthquakes. Bearing isolation systems have in somecircumstances, e.g. the 1994 Northridge (Calif.) earthquake,demonstrated to provide poor if any base isolation of the structure fromthe earthquake induced ground motion. The mass damping systems havedemonstrated some protection of a structure due to earthquakevibrations; however, they are expensive, and difficult to implement inexisting structures. The energy absorbing sliding bearing systems can beimplemented in existing structures; however, their performance duringactual earthquake events appear limited in isolating the structure fromearthquake induced ground motions and minimizing structural damage.

The main forces acting on soil elements in the field during earthquakesare those resulting from the upward migration of shear motions from theunderlying rock formations. Although the actual wave pattern may be verycomplex, the resulting repeated and reversing shearing deformations,imposed on the soil by the S-wave components are the principal cause ofa phenomenon known as liquefaction, which occurs in saturated fine sand,silty sand and silt deposits. When these soil deposits are subjected torepeated shear strain reversals, the volume of the soil decreases witheach cycle, i.e. the soil contracts, and due to the lack of drainage ofthese saturated soils, the soil pore water pressure rises. As the soilpore water pressure rises, the grain to grain contact pressure becomessmaller, until eventually the grain to grain contact pressure drops tozero and the soil loses all of its shear strength and acts like a fluid.Liquefaction can occur in loose saturated fine sands, silty sands andsilts as a result of earthquakes, blasting or other shocks.

The factors that effect the occurrence of liquefaction are soil type,grain size distribution, compactness of the soil, soil permeability, andthe magnitude and number of the strain reversals. Fine cohesionlesssoils, fine sand or fine cohesionless soils containing moderate amountsof silt are most susceptible to liquefaction. Uniformly graded soils aremore susceptible to liquefaction than well graded soils, and fine sandstend to liquefy more easily than coarse sands or gravelly soils.Moderate amounts of silt appear to increase the liquefactionsusceptibility of fine sands; however, fine sands with large amounts ofsilt are less susceptible, although liquefaction is still possible.Recent evidence indicates that sands containing moderate amounts of claymay also be liquefiable.

Current methods for evaluating the liquefaction potential of soilsconsist of two basic approaches, laboratory tests and in situ tests. Thelaboratory methods require undisturbed soil samples which are difficultto impossible to obtain. The laboratory test methods involve cyclictriaxial, cyclic direct shear and cyclic torsional triaxial tests All ofthese tests apply a cyclic shear stress reversal upon the soil specimen.At the present time, there is not a method for obtaining undisturbedsamples, in which the in situ stress state, void ratio or structure havebeen preserved in cohesionless soils. Therefore, laboratory methods areconsidered only qualitative tests in assessing the potential of a soilto liquefy. The in situ methods currently consist of five (5) types,with four (4) of the methods; 1) the Standard Penetration Test (SPT); 2)the Cone Penetration Test (CPT); 3) the Piezocone Penetration Test(PCPT) and 4) the Seismic Waves Test (SWT) being indirect empiricalmethods and the fifth method an in situ cyclic stress reversal testbeing a direct in situ measurement of a soil's tendency to liquefy. Thepresent in situ methods are capable of determining whether a particularsoil horizon has the potential to liquefy and under what earthquakeground motions it will most likely liquefy.

Since shear waves can not propagate through a fluid, a liquefied soilhorizon will act as a seismic isolation barrier and stop/inhibit theupward propagation of earthquake induced shear wave ground motions tooverlying soils and structures. To avoid liquefaction related damage tothe surface the liquefied soil horizon needs to be at a depth greaterthan 5 times its liquefiable thickness, Ishihara (1985) and Youd &Garris (1995).

Electro-osmosis involves the application of a direct current (dc)between electrodes inserted in the saturated soil, that gives rise topore fluid movement from the source electrodes towards the sinkelectrodes and thus modifies the soil pore water pressures.Electro-osmosis has been used in applications such as 1) improvingstability of excavations, 2) decreasing pile driving resistance, 3)increasing pile strength, 4) stabilization of soils by consolidation orgrouting, 5) dewatering of sludges, 6) groundwater lowering and barriersystems, 7) increasing petroleum production, 8) removing contaminantsfrom soils, and 9) preventing soil liquefaction during an earthquakeevent. Electro-osmosis uses a dc electrical potential difference appliedacross the saturated soil mass by electrodes placed in an open or closedflow arrangement. The dc potential difference sets up a dc currentflowing from the source electrodes to the sink electrodes. In most soilsthe soil particles have a negative charge. For those negatively chargedsoils, the source electrodes is the anode electrode, the sink electrodeis the cathode electrode, and ground water migrates from the anodeelectrode towards the cathode electrode. In other soils, such ascalcareous soils (e.g. limestone), the soil particles carry a positivecharge. In those positively charged soils, the source electrode is thecathode electrode, the sink electrode is the anode electrode, and groundwater migrates from the cathode electrode towards the anode electrode.

An “open” flow arrangement at the electrodes allows an ingress or egressof the pore fluid. Due to the electrically induced transport of porewater fluid, the soil pore water pressures are modified to enableexcavations to be stabilized or pile driving resistance to be lowered.Electro-osmosis is not used extensively due to the high cost ofmaintaining the dc potential over long periods of time and the dryingout and chemical reactions that occur if the system is activated forlong periods of time.

Monitoring ground motion and activating safety devices or active massdamping systems prior to the arrival of a major earthquake can in somecases reduce damage. Such a forecasting system can be used to close gasvalves or cutoff electricity to the effected area. Such systems mayinclude a tuned pendulum system, that upon the onset of certain groundmotion magnitude and frequency, the pendulum motion sets off an alarm,activates a switch or closes a gas valve prior to the arrival of themajor tremor of the earthquake. Alternatively, a heavy sliding orrotating mass can be used to activate a similar switch, contact orvalue, by sizing the mass that upon experiencing certain ground motionsthe mass slides or rotates and activates a switch, contact or closes avalve prior to the arrival of the major destructive earthquake tremor.

SUMMARY OF THE INVENTION

The present invention provides a method and system for seismicallyisolating a structure or works from earthquake induced ground motionsduring an earthquake event.

Particularly, the present invention provides a seismic monitor thatmonitors the earth's movement and predicts the onset of an earthquakeevent. Based on that prediction, the methods and system of the presentinvention controls a switch that activates a dc potential differenceacross an array of electrodes buried in the ground beneath the structureand below the water table. The current flow by means of electro-osmosisraises the pore water pressure in a particular predetermined isolationlayer of the soil horizon to such an extent that the soil horizonpreferentially temporarily liquefies within that isolation layer duringthe earthquake event. The liquefied isolation layer of the soil horizonisolates the overlying soil and structure from the upward propagatingearthquake induced shear wave ground motions. The electrodes arespatially located in the saturated soil beneath the structure to definethe size of the isolation layer and to induce ground water flow towardsthe isolation layer of the soil horizon with the sink electrodes locatedin or adjacent to the isolation layer of the soil horizon. The spatiallocations of the electrodes and the applied dc potential difference willvary depending on the soil conditions and the structure, but the dcpotential needs to be sufficiently effective to raise the pore waterpressure within the particular isolation layer of the soil horizonsufficiently high to ensure the soil horizon liquefies in the isolationlayer during the earthquake event.

The particular isolation layer of the soil horizon selected for inducedliquefaction is identified from in situ tests and the thickness of theliquefied isolation layer of the soil horizon is established by thevertical spatial location of the electrodes. The vertical thickness ofthe liquefied isolation layer is selected to be less than ⅕^(th) (20%)of the depth of the liquefied isolation layer to ensure liquefactionrelated surface damage does not occur. The potential isolation layer ofthe soil horizon is selected as the most susceptible soil in theformation that will liquefy and is conducive to electo-osmosis. Theisolation layer of the soil horizon is also selected based on its low orminimal cohesive strength.

Particularly, the method of the present invention raises the pore waterpressure in the isolation layer of the soil horizon by activating anelectro-osmosis gradient towards the isolation layer to ensure that theisolation layer of the soil horizon liquefies during the earthquakeevent and thus the liquefied isolation layer seismically isolates theoverlying soil and structure from the upward propagating earthquakeinduced shear wave ground motions. The present invention can beinstalled for existing structures with minimal disruption and can reducethe damaging vibrations to the structure and its foundations byisolating the structure and foundations from the earthquake event.

While the liquefied isolation layer, well below the overlying structure,provides a high degree of protection for the overlying structure, insome circumstances electro-osmosis may also be used directly beneath theoverlying structure and above the isolation layer to inhibit soilliquefaction directly beneath the overlying structure to maintain asolid soil foundation zone during the earthquake event. Inhibiting soilliquefaction directly beneath the overlying structure during anearthquake event is described in commonly owned U.S. Pat. No. 6,308,135.Therefore, the present invention contemplates the combination ofinhibiting soil liquefaction directly beneath the overlying structure tomaintain a solid soil foundations zone while at the same time inducingsoil liquefaction in an isolation layer at a greater depth in order toisolate the overlying soil and structure from the upward propagatingearthquake induced shear wave ground motions.

A seismic monitor can consist of a variety of devices provided they canpredict the onset of major shear deforming ground motions associatedwith the major earthquake tremor from either early time arrival ofhigher frequency ground motions or the onset of strong ground motions.The seismic monitor may comprise an accelerometer connected to acomputer running a predictive algorithm to activate the switch if groundmotions of certain magnitude and frequency are experienced. The seismicmonitor may also comprise a pendulum tuned to either activate ordeactivate a contact if ground motions of certain magnitude andfrequency are experienced. The seismic monitor may further comprise asliding or rotating mass of sufficient mass to activate or deactivate acontact if ground motions of a certain magnitude and frequency areexperienced. The seismic monitor in all cases is designed to monitorground movement and based on that ground movement predict the onset of amajor earthquake tremor. Such seismic monitors are fully disclosed incommonly owned U.S. Pat. No. 6,308,135.

When the seismic monitor has predicted the onset of a major earthquaketremor, the seismic monitor actuates a switch that connects the dc powersource to an array of electrical electrodes in the saturated ground, toinduce ground water flow from the source electrodes to the sinkelectrodes and raise the pore water pressure in a particular soilhorizon sufficiently high to ensure the isolation layer of the soilhorizon liquefies during the earthquake event. The seismic monitor'sprediction of a major earthquake tremor generally only precedes themajor earthquake tremor by a few seconds, so the dc power source must becapable of energizing the electrodes within this time frame. Such dcpower sources may include lead acid batteries, a fly wheel generator, aquick start gas or diesel powered generator, or a combination thereof.Based on empirical data, at least 7.5 watts per square foot of theoverlying structure is typically required to induce soil liquefaction inthe isolation layer.

Upon energizing of the electrodes a timer is also activated. The timeris set to disengage the electrodes from the dc power source only aftersufficient time to ensure the electrodes remain energized throughouteven the longest previously recorded earthquake duration. Uponde-energizing the electrodes, the system is reset, and the seismicmonitor can re-activate and reenergize the electrodes in the event offollowing earthquake tremors. The dc power source needs to be ofsufficient capacity or re-chargeable to energize the electrodes at thepower requirements and duration to ensure the isolation layer of thesoil horizon remains in a liquefied state during the earthquake event.

Following the earthquake event, the potential difference applied acrossthe buried electrodes can be reversed to quickly dissipate the excesspore water pressures generated within the liquefied isolation layer ofthe soil horizon. During this potential difference reversal, water isextracted from the original source electrodes/water supply wells tohasten the dissipation of the pore water pressures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing one form of the invention forraising the pore water pressure in a particular isolation layer of thesoil horizon beneath a structure during an earthquake event.

FIG. 2 is a grain size distribution envelope of a range of soilsapplicable to the current invention.

FIG. 3 is a cross sectional view showing another form of the inventionwith the source electrodes (e.g. anode electrodes) also being watersupply wells for raising pore water pressures in a particular isolationlayer of the soil horizon beneath a structure during an earthquakeevent.

FIG. 4 is a cross-sectional view of source electrodes/supply wells (e.g.anode electrodes) and sink electrodes (e.g. cathode electrodes) as givenin FIG. 3.

FIG. 5 is a cross sectional view showing another form of the inventionfor raising the pore water pressure in a particular isolation layer ofthe soil horizon well beneath a structure during an earthquake eventwhile at the same time lowering the pore water pressure directly underthe structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method and system for seismicallyisolating a structure or works from earthquake induced ground motions.The method and system of the present invention employes a seismicmonitor which activates a electro-osmosis system in the sub-surfacesaturated soils well beneath the structure. The electro-osmosis systemraises the pore water pressure within a particular isolation layer ofthe soil horizon to ensure that soil horizon within that isolation layerliquefies during the earthquake event and thus isolates the overlyingstructure from the earthquake induced upward propagating shear waveground motions. One form of the invention is illustrated in crosssection in FIG. 1, with a structure 1 constructed on a saturated soil 2which is prone to amplify earthquake induced ground motions. Aseismically triggered switch 3 comprises an accelerometer 4 andpredictor algorithm 5 running on a computer. The accelerometer 4 engagesthe ground surface 24 in order to sense any movement of the groundsurface 24. The predictor algorithm 5 monitors the accelerometer outputand predicts the onset of a major earthquake tremor from early arrivalground motion from minor earthquake tremors. In addition to theaccelerometer 4 and predictor algorithm 5, The seismically triggeredswitch 3 may alternatively comprise a pendulum tuned to either activateor deactivate a contact if ground motions of certain magnitude andfrequency are experienced. The seismically triggered switch 3 may alsocomprise a sliding or rotating mass of sufficient mass to activate ordeactivate a contact if ground motions of a certain magnitude andfrequency are experienced. The seismically triggered switch 3 in allcases is designed to monitor ground movement, and based on that groundmovement, to predict the onset of a major earthquake tremor. Thearrangement and operation of the seismically triggered switch 3 isdisclosed in greater detail in commonly owned U.S. Pat. No. 6,308,135.

The output of the predictor algorithm 5 is connected to a switch 6 thatconnects a dc power source 7 to an array of electrical horizontalconductors, both positive source conductors 8 and negative sinkconductors 9 positioned in and around a particular isolation layer 10 ofthe soil horizon which is susceptible to liquefaction. The conductors 8and 9 remain energized throughout the major earthquake tremor by the useof a timer 11 which is activated by the seismically triggered switch 6.The timer 11 de-energizes the conductors 8 and 9 after an elapsed timeand re-sets the switch 6 and the algorithm 5, so that the system can bere-triggered in the event of a later earthquake or tremor.

With respect to the array of electrical conductors, the positive sourceconductors 8 are connected to the positive terminal of the dc powersource 7, and the negative sink conductors 9 are connected to thenegative terminal of the dc power source 7. The positive sourceconductors 8 are located above and below the isolation layer 10, and thenegative sink conductors 9 are located within the isolation layer 10.When the dc power source 7 is connected to the conductors 8 and 9, theground water flows from the positive source conductors 8 towards thenegative sink conductors 9 whereby raising the soil pore water pressurein the isolation layer 10 of the soil horizon. The increased soil porewater pressure within the isolation layer 10 preferentially inducesliquefaction of the soil within the isolation layer 10. The liquefiedisolation layer 10 beneath the structure 1 thus isolates the structure 1and the soil 2 directly beneath the structure 1 from the upwardpropagating earthquake induced shear wave ground motions.

With continuing reference to FIG. 1, the horizontal positive sourceconductors 8 are spaced vertically from each other to define a thickness20 for the isolation layer 10. As can be seen in FIG. 1, the isolationlayer 10 is located a distance 22 below the ground surface 24. In orderto assure proper isolation by the isolation layer 10 and to avoidliquefaction related damage to the surface and overlying structure 1,the thickness 20 of the isolation layer 10 is preferably less than 20%of the depth 22 of the isolation layer 10.

The present invention is applicable to an isolation layer 10 consistingof fine grained saturated soils, such as fine sands, silty sands, siltsand clayey soils. The grain size distribution envelope of soilssusceptible to liquefaction are shown in FIG. 2. The soils with a grainsize distribution that lies within the envelope 13 are susceptible tosoil liquefaction during an earthquake event. The soils applicable toelectro-osmosis and susceptible to soil liquefaction during anearthquake event are generally contained in the grain size distributionenvelope 14. The present invention is applicable to these soils whichare classified as d10 (10% finer) being less or equal to a grain size of0.05 mm as shown by 15 in FIG. 2. That is, 10% by weight of the soil hasa grain size equal to or less than 0.05 mm. The particular soil horizon10 needs to be susceptible to liquefaction, whereas soil 2 does notnecessarily need to be readily liquefiable.

Referring now to FIG. 3 and FIG. 4 a further embodiment is shown inwhich the parts corresponding to those in FIG. 1 are identical andsimilarly numbered with the exception of the electrical conductors,positive source conductors 16 and negative sink conductors 17.Particularly, the source conductors 16 are located and connected towater supply wells 18. In this form of the invention, the ground wateris driven by the electro-osmotic gradient from the source conductors 16towards the sink conductors 17 located in the isolation layer 10 of thesoil horizon. The water supply wells 18 supply additional water to thesource conductors 16 to assist in raising the pore water pressure in theisolation layer 10 of the soil horizon. The soil pore water pressure inthe isolation layer 10 of the soil horizon beneath the structure will bemost effectively increased by this arrangement to result in thepreferential liquefaction of the isolation layer 10 of the soil horizonduring a moderate to large earthquake event. The temporary liquefactionof the isolation layer 10 of the soil horizon during the earthquakeevent results in the isolation layer 10 of the soil horizon acting likea fluid, and thus upward propagation of earthquake induced shear groundmotions are not transmitted through this liquefied isolation layer 10,and the overlying structure 1 is isolated from such ground motions. Thewater supply wells 18 are connected to the source conductors 16, whichare porous and are able to transmit the necessary volumes of waterrequired to ensure an adequate pore water pressure rise in the isolationlayer 10 to induce liquefaction within this soil horizon.

In order to achieve adequate liquefaction of the isolation layer 10 andthereby achieve adequate isolation of the overlying structure 1 from theupwardly propagating shock waves from an earthquake, a sufficient amountof electrical energy must be imparted to the isolation layer 10.Experimental results indicated that at least 7.5 watts per square footof space occupied by the overlying structure 1 is required.

Turning to FIG. 5, a further embodiment of the present invention isshown. The embodiment of the present invention shown in FIG. 5 issimilar in most respects to the embodiment shown in FIG. 1, except thata second array of electrical conductors including positive sourceconductors 26 and negative sink conductors 28 are located in the soilhorizon below the water table 12 and above the isolation layer 10.Particularly, the positive source conductors 26 are located in the soilhorizon below the water table 12, above the isolation layer 10, anddirectly beneath the structure 1. On the other hand, the negative sinkconductors 28 are located below the water table 12, above the isolationlayer 10, and outboard of the structure 1. Consequently, when the dcpotential from the dc power supply 7 is connected to the conductors 26and 28, ground water flows from the positive source conductors 26 towardthe negative sink conductors 28 thereby lowering the soil pore waterpressure in the soil 2 directly beneath the structure 1. The reducedsoil pore water pressure beneath the structure 1 inhibits liquefactionof the soil 2 in response to shock waves directly beneath the structure1 thereby maintaining a firm soil foundation zone 30 under the structure1 during an earthquake event. The soil foundation zone 30 firmlysupports the structure 1 while the isolation layer 10 isolates thefoundation zone 30 and the overlying structure 1 from upwardlypropagating shock waves.

There is a plurality of arrangements and positions of the electricalconductors and water supply wells to achieve the desired soil pore waterpressure increase by electro-osmosis in the isolation layer beneath thestructure founded on saturated soils. The above arrangements are shownas illustrations of various forms of the invention. The presentinvention, therefore, is well adapted to carry out the objects andattain the ends and advantages mentioned as well as others inherentherein. While presently preferred embodiments of the invention are givenfor the purpose of disclosure, numerous changes in the details ofconstruction, arrangement of parts, and the steps of the process willreadily suggest themselves to those skilled in the art and which areencompassed within the spirit of the invention and the scope of theappended claims.

I claim:
 1. A method of seismically isolating a structure on a groundsurface during an earthquake event comprising: (a) monitoring themovements of the earth adjacent the structure; (b) from the movements ofthe earth, predicting the onset of an earthquake event; and (c) inresponse to the prediction of an earthquake event, connecting a sourceof dc power to an array of electrical conductors located in the soilbeneath the structure and below the ground water table so that inresponse to connecting the dc power to the electrical conductors in thesoil, the ground water moves towards an isolation layer, raising thepore water pressure within the isolation layer and thus causingpreferential earthquake induced liquefaction of the isolation layerwhich thus isolates the overlying structure from the upwardlypropagating earthquake induced ground motions.
 2. The method of claim 1,wherein at least one of the conductors is located in or adjacent to awater supply well to provide the additional amounts of water required tomigrate towards the other electrical conductors located in theliquefiable isolation layer to ensure the pore water pressure in theisolation layer raises sufficiently high to induce liquefaction of theisolation layer beneath the structure during the earthquake event. 3.The method of claim 1, wherein at least one of the conductors is porousor open to transmit water and is connected to a supply of water toprovide the additional amounts of water required to migrate towards theother electrical conductors located in the liquefiable isolation layerto ensure the pore water pressure in the isolation layer raisessufficiently high to induce liquefaction of the isolation layer beneaththe structure during the earthquake event.
 4. The method of claim 1,wherein the method further comprises maintaining the connection of thedc power source to the array of electrical conductors for apredetermined time relating to the expected duration of the earthquakeevent.
 5. The method of claim 1, wherein the earth's movement ismonitored by an accelerometer which produces output signalscorresponding to the earth's movement and the onset of an earthquakeevent is predicted by means of a computer running an algorithm using theoutput signals.
 6. The method of claim 1, wherein the earth's movementis monitored and the onset of earthquake events is predicted by amechanical device comprising a pendulum mass that actuates orde-actuates a switch when subjected to early arrival ground motionspreceding the onset of an earthquake event.
 7. The method of claim 1,wherein the earth's movement is monitored and the onset of earthquakeevents is predicted by a mechanical device comprising a sliding massthat actuates or de-actuates a switch when subjected to early arrivalground motions preceding the onset of an earthquake event.
 8. The methodof claim 1, wherein the earth's movement is monitored and the onset ofearthquake events is predicted by a mechanical device consisting of arotating/rolling mass that actuates or de-actuates a switch whensubjected to early arrival ground motions preceding the onset of anearthquake event.
 9. The method of claim 1, wherein the isolation layeris located at a depth below the ground surface and has a thickness thatis equal to less than ⅕ of the depth of the isolation layer below theground surface.
 10. The method of claim 1, wherein the soil within theisolation layer consists of fine grained saturated soils, such as finesands, silty sands, silts and clayey soils.
 11. The method of claim 10,wherein the soil within the isolation layer consists of soils that areclassified as d10 (10% finer) being less or equal to a grain size of0.05 mm.
 12. The method of claim 1, wherein the method further comprisesconnecting the source of dc power to a second array of electricalconductors located in the soil beneath the structure, below the groundwater table, and above the isolation layer so that in response toconnecting the dc power to the second array of electrical conductors inthe soil, the ground water moves away from beneath the structure, thepore water pressure beneath the structure is lowered, and preferentialearthquake induced liquefaction of the soil beneath the structure isinhibited.
 13. A system of seismically isolating a structure on theground surface during an earthquake event comprising: (a) a dc powersource; (b) an array of electrical conductors located in the soilbeneath the structure and below the ground water table; (c) a switchinterconnecting the dc power source and the array of electricalconductors; and (d) a seismic monitor for monitoring the movement of theearth adjacent the structure, and from the movement, predicting theonset of an earthquake event, wherein the seismic monitor activates theswitch in response to predicting the onset of an earthquake event sothat the dc power source is connected to the conductors thereby causingthe ground water to move towards an isolation layer beneath thestructure, raising the pore water pressure within the isolation layer toensure the isolation layer liquefies during the earthquake event andthus isolates the structure from the upwardly propagating earthquakeinduced ground motions.
 14. The system of claim 13, wherein at least oneof the conductors is located in or adjacent to a water supply well toprovide the additional amounts of water required to migrate towards theother electrical conductors located in the liquefiable isolation layerto ensure the pore water pressure in the isolation layer raisessufficiently high to induce liquefaction of the isolation layer beneaththe structure during the earthquake event.
 15. The system of claim 13,wherein at least one of the conductors is porous or open to transmitwater and is connected to a supply of water to provide the additionalamounts of water required to migrate towards the other electricalconductors located in the liquefiable isolation layer to ensure the porewater pressure in the isolation layer raises sufficiently high to induceliquefaction of the isolation layer beneath the structure during theearthquake event.
 16. The system of claim 13, wherein the system furthercomprises a timer, connected between the seismic monitor and the switch,which timer is activated by the seismic monitor in response to theprediction of the onset of an earthquake event, and which timermaintains the connection of the dc power source to the array ofelectrical conductors for a predetermined time relating the expectedduration of the earthquake event.
 17. The system of claim 13, whereinthe seismic monitor comprises an accelerometer for monitoring theearth's movement and for producing output signals corresponding to suchmovement and a computer adapted to run an algorithm which uses theoutput signals to predict the onset of an earthquake event.
 18. Thesystem of claim 13, wherein the seismic monitor comprises a mechanicaldevice comprising a pendulum mass that actuates or de-actuates a switchwhen subjected to early arrival ground motions preceding the onset of anearthquake event.
 19. The system of claim 13, wherein the seismicmonitor comprises a mechanical device comprising a sliding mass thatactuates or de-actuates a switch when subjected to early arrival groundmotions preceding the onset of an earthquake event.
 20. The system ofclaim 13, wherein the seismic monitor comprises a mechanical devicecomprising a rotating/rolling mass that actuates or de-actuates a switchwhen subjected to early arrival ground motions preceding the onset of anearthquake event.
 21. The system of claim 13, wherein the first array ofconductors opposition to define the boundaries of the isolation layerwhich is thereby located at a depth below the ground surface and has athickness that is less than ⅕ of the depth of the isolation layer belowthe ground surface.
 22. The system of claim 13, wherein the soil withinthe isolation layer consists of fine grained saturated soils, such asfine sands, silty sands, silts and clayey soils.
 23. The system of claim22, wherein the soil within the isolation layer consists of soils thatare classified as d10 (10% finer) being less or equal to a grain size of0.05 mm.
 24. The system of claim 13, wherein the system furthercomprises a second array of electrical conductors connected to thesource of dc power and located in the soil beneath the structure, belowthe ground water table, and above the isolation layer so that inresponse to connecting the dc power to the second array of electricalconductors in the soil, the ground water moves away from beneath thestructure, the pore water pressure beneath the structure is lowered, andpreferential earthquake induced liquefaction of the soil beneath thestructure is inhibited.